60 Sidereal Explorations
complex of systematic motion which is not yet satisfactorily
interpreted.
The refinement of the star counts and their extension to
faint magnitudes, especially at the Mount Wilson and
Groningen observatories, has carried far our knowledge of
stellar distribution in the solar neighborhood. But the study
of the distribution of the brighter stars of each spectral
type has done more to bring to light the existence of a
“local system.” Formerly this local star cloud was not dis-
tinguished from the general galactic system, and its boun-
daries are so poorly known that we cannot now feel entirely
sure of its distinctness from surrounding galactic clouds or
even of its organization as a separate stellar system.
Nevertheless, the common opinion is that a local star
cloud surrounds the Sun; its diameter is a few thousand
light years ; its population is probably numbered in millions
of stars; its membership includes most of the naked-eye
stars and many of the conspicuous clusterings such as the
Pleiades, the Hyades, and the constellations of high tem-
perature stars in Orion, Scorpio, and Centaurus. Less com-
mon is adherence to the provisional view of the writer that
the motions of stars within the local system, and the mo-
tions of the system as a whole with respect to other star
clouds of the Milky Way, give rise to the phenomenon fre-
quently interpreted as galactic rotation. Our studies of the
proper motions and radial velocities of stars deal with such
a localized region near the Sun that I feel it incautious to
extrapolate from a meager amount of material and form the
hypothesis that the galactic system rotates as a single unit.
The galactic system is perhaps a thousand times as large
as the region so far explored for stellar motions.
But it may be well for the present to postpone considera-
tion of the incompatibility between simple galactic rotation